The Voids

In hardened HCC, a void is an empty space, other than a crack, in the cement paste
that contains nothing but air. The type, size, shape, arrangement, and abundance
of the voids are factors controlling many important properties, such as compressive
strength, resistance to destruction by freezing and thawing, and resistance to chemical attack on the reinforcing steel and the cement paste. The percentage of air void
volume is generally specified by the design of the mixture. A large number of very
small (most not visible without magnification) air voids are desired so that an appropriate
amount of air can be distributed throughout the HCC in such a way that the distance
between voids is very short and, thus, the paste is protected from freezing and thawing.
A ratio of air-void volume to paste volume that exceeds the specified range weakens the
HCC and may create channelways for the permeation and circulation of deleterious substances.

The total air-void content (of voids larger than capillary size) of an unhardened
concrete mixture is routinely determined in accordance with ASTM C 231 (the
pressure method) or ASTM C 173 (the volumetric method). Unit weight determinations
(ASTM C 138) may also be determined to provide information concerning the percentage
of air in the mixture. These methods do not ascertain the type of voids present;
they merely measure the total void content. These measurements are important.
As Bartel (1978) stated:

Tests for air content and unit weight of fresh concrete, carefully made in accordance with the
appropriate ASTM test method, will yield an accurate measurement of the amount of air, weight, and
volume of concrete being produced. Tests for air content, coupled with intelligently selected
specification limits, can ensure the beneficial effects of entrained air in hardened concrete (p. 130).

Specially formed specimens of hardened HCC mixtures may be tested for resistance
to the destructive forces of freezing and thawing in accordance with ASTM C
666 (resistance to rapid freezing and thawing). An HCC that is resistant generally
indicates that an adequate air-void system is present or that the HCC has not
become critically saturated.

It has been variously claimed that the total air-void content increases or decreases
as the concrete hardens. It appears that what really happens is that the determination
of total air-void content with field equipment made on the fresh concrete does not agree
with the total air-void content determined by microscopical analysis of the hardened
concrete. Except in the case of hydrogen gas being evolved by the incorporation of
aluminum fragments (Newlon & Ozol, 1972) (see Fig. 6-1), no evidence of an equivalent
change in the volume of concrete in the field placement or test cylinders has been offered
as evidence corroborating the increase or decrease in total air-void content.

Careful investigation by a combination of controlled mixing and sampling procedures and
petrographic techniques has shown that the air-void content does not change on hardening
but rather may change due to outside influences, such as extreme overconsolidation
(unusually long vibration, thus removal of more of the entrapped air than usual) or the
further addition of water and retempering. The determination of the void

Figure 6-1 CONCRETE THAT INCREASED IN VOLUME DUE TO INCORPORATION OF
ALUMINUM FRAGMENTS. Since it was cast in the cylinder mold, the concrete increased in
volume due to the incorporation of aluminum fragments (from an aluminum delivery pipe). Thus,
hydrogen gas evolved from the chemical reaction of the aluminum with the alkaline fluids of the
fresh cement paste.

content in the hardened state should agree within 1 percent
with the void content determined in the fresh, unhardened state. When they do not
closely agree, either one of the measurements is in error or the two specimens
tested do not represent the same concrete subjected to the same influences (see
Appendix D and Ozyildirim, 1991).

An air-void content in excess of the amount required for protection against the
destructive forces of alternate freezing and thawing that occurs in saturated concrete
adds no benefit to concrete expected to bear loads and resist abrasion. (For a
discussion of the high-air, cellular concretes, see Lesatski [1978] and Lewis [1978].)
An excessive air-void content will lower the compressive strength of the concrete by
about 5% for each excess percentage of voids.

In the early days of the use of air-entraining agents, there were some who saw that
cracks ended at air voids and interpreted this to mean that the cracks started at air
voids and were caused by them. In many parts of the United States, there was (and
still is) a fear of an air-void content that exceeded the minimum specified. This may
be interpreted as a fear of not meeting the compressive strength requirements. By
nature of the bell-shaped probability curve, avoidance of high air-void contents can
lead to air contents that are below the amount required for resistance to the destructive
forces of freezing and thawing in saturated concrete. Conversely, there
now seems to be, at least in this area of the United States among some concreting
contractors, a fear of low air content. When HCC containing a percentage of air
near the top limit of the specified amount is altered by the addition of retempering
water (which can cause air-entraiing agents to be more active), an HCC with an
air-void content considerably higher than the specified quantity is frequently produced:
the result is concrete of insufficient strength (see Appendix D).

6.2 TYPES OF VOIDS

The overall void content in HCC is composed of four general types of voids, as listed
in Table 6-1.

Table 6-1
TYPES OF VOIDS

1. Capillary voids. Capillary voids are irregularly shaped and very small, less than 5 µm on the
lapped surface of the slice examined. They represent space originally filled by mixing water,
remain after the hydration of the cement gels, and are an integral part of the paste. Although
they contain air at the time of examination, they are not considered part of the air void system.

2. Entrained air voids. Entrained air voids are defined at VTRC as spherical voids larger than
the capillaries but less than 1 mm on the lapped surface of the slice examined. They are formed
by the folding action of the concrete mixer, and their shape and size and abundance are influenced
by the addition of surface-active air-entraining admixtures to the mixture.

3. Entrapped air voids. Entrapped air voids are voids that are larger than the entrained voids
but have internal surfaces that indicate they were formed by air bubbles or pockets. They may
be spherical or irregularly shaped.

4. Water voids. Water voids are irregularly shaped voids whose shape, location, or internal surface
indicates that they were formed by water. Usually, they are larger than entrained air voids.

6.2.1 Capillary Voids

The smallest class of optically visible voids in HCC are the various sizes of capillaries.
A very few of the larger capillary voids may be seen at the higher magnifications used
to determine the parameters of the void system, but they are generally not that large.
Capillary voids are spaces formed by the shape of the hydrated cement gel structures
and spaces left between the gel structures as water is used in
the self-desiccation of the hydration process. They were occupied by water or gas
when the concrete was fresh and are larger and more abundant in concretes with a
high water-cement ratio. The magnitude of the capillary system is controlled by the
water-cement ratio and the degree of maturity of the concrete. The evenness of the
distribution of the pores and capillaries is controlled by the distribution of the
water. As the concrete hydrates, the water in the pores is used in the hydration of
the cement. As the concrete matures, much of the capillary space becomes filled
with the products of hydration and the products of any reactions occurring between
the chemicals of the paste and the aggregate rocks. Some of the finer capillaries are
spaces created by differential crystal growth. (See Chapter 13 and associated figures
and note how the quality of the fine aggregate affects the distribution of moisture
and thus pores and capillaries in the paste.)

The capillaries are detected only when specialized methods are used. In laboratories
so equipped, the various types of electron microscopes may be used to view the
capillary void system. In the VTRC laboratory, the abundance and location of the
capillary voids are detected by use of the P/EF microscope in the study of fluorescent
thin sections of the specimen concrete (see Chapter 13). Rarely, capillary voids
may be noted during the determination of the parameters of the void system. In
that event, capillary voids are considered to be paste.

6.2.2 Entrained Air Voids

Entrained voids are small spherical voids enfolded by the mixer. Surface-active,
air-entraining agents are added to the mixture to stabilize a specified percentage of
these voids and thus protect the hardened HCC against the destructive forces of
freezing and thawing. Thus, the entrained air void is a desirable void. Entrained
air voids are generally considered to be larger than the capillaries (at least 5 µm in
diameter) but smaller than the voids considered to be entrapped voids (Verbeck,
1966, 1978). Entrained air voids have so much surface tension relative to their volume
that they are distorted very little by the shape of nearby particles. Distortion
occurs in these small voids only when external forces distort the concrete after the
beginning of hardening.

The presence of the proper quantity of well-distributed entrained air voids can prevent
deterioration of the concrete (even when saturated) by the mechanisms of
freezing and thawing (Helms, 1978; Newlon, 1978) and facilitate the placement of
the concrete because the entrained air voids act as additional fluid, almost as
though the entrained voids were ball bearings. In Virginia, the proper quantity of
air voids is defined for each class of HCC by Road and Bridge Specifications (1991).
Entrained air voids allow the relief of pressures caused by the freezing and thawing
of saturated HCC and thus protect the HCC from destruction by these mechanisms.
The exact method by which they perform this function has not been determined and
agreed on by all concrete technologists, but all agree that the empirical evidence
demonstrates that the presence of a sufficient quantity of sufficiently small (entrained
size), properly distributed air voids protects the cement paste in the concrete from
deterioration by freezing and thawing.

Very irregularly shaped small voids (maximum dimension less than 1 mm) cannot
be properly called entrained voids because the surface tension caused by the air entraining
agent is lacking. It is not known if such voids function to protect the
concrete against the deterioration caused by freezing and thawing. Small, irregular
voids, particularly if not at an aggregate boundary or a wearing surface, may be evidence
of retempering (see Appendix D).

Figures 6-2 and 6-3 show varying percentages of air voids.

6.2.3 Entrapped Voids and Water Voids

All voids, regardless of shape, that have a maximum dimension (on the surface examined)
of more than 1 mm are defined at VTRC as entrapped voids (large spherical)
or water voids (large irregular). If voids occur flattened out at the boundary between
the aggregate (usually coarse aggregate) and the paste, they are a class of
entrapped voids called boundary voids.

All voids larger than entrained voids have no appreciable beneficial effects and
weaken the HCC. Such voids are partially controlled by the efficiency of whatever
system of consolidation is in use. Certain voids may be caused by too much water in
the HCC, a strong affinity of a particular aggregate lithology for water, improper
consolidation, and occasionally by the dissolving away of Ca(OH)2. Irregularly
shaped voids, regardless of size, may be caused by water pockets or air pockets that

Figure 6-2 SURFACE OF FINELY LAPPED SLICE OF CONCRETE CONTAINING 5.6%
TOTAL AIR VOIDS. The void content of this concrete is in the middle of the specification range.
The large void marked with an arrow is about 2 mm across (larger than an entrained air void).
Notice the very fine voids throughout the paste.

Figure 6-3 SURFACE OF FINELY LAPPED SLICE OF CONCRETE CONTAINING 17%
TOTAL AIR VOIDS. The void content of this concrete is way above the upper limit of the
specification range. The void indicated by the arrow is about 1 mm across. The area of darker paste
(lower left) has a lower void content. An HCC that contains more than one kind of paste generally
indicates that the mixture had begun to hydrate before additional water was added (see 8.4 and
Appendix D).

Figure 6-4 CONCRETE CORE WITH ABOUT 4% LARGE IRREGULAR VOIDS. In this
instance, the concrete, which had not yet been consolidated, became hard and unworkable while
repairs were being made on the paving equipment.

the consolidation procedures did not remove (see Fig. 6-4). Water voids are irregularly
shaped voids created in the HCC by bleed water prevented from rising to the
surface by an aggregate particle or the hardening of the paste. Water voids contained
water when the HCC was fresh and unhardened. In the hardened state, these voids are
filled with air and might be more properly termed water-formed air voids.

6.3 QUANTITATIVE DETERMINATION OF AIR-VOID PARAMETERS

6.3.1 Overview

In hardened concrete, the parameters of the air-void system may be determined by
obtaining the data and performing the calculations specified in ASTM C 457. The
parameters calculated include:

1. Air-void content (symbolized in ASTM C 457 by A). It is a percentage by
volume. A minimum amount of air voids are required to protect the concrete from
the expansion of water during freezing. Excess air-void content will cause the
concrete to have less than the intended compressive strength.

2. Void frequency (symbolized in ASTM C 457 by n.) It is the number of voids per
unit length of traverse. The void frequency is required in the calculation of the
average chord in the modified point-count method.

3. Average chord length (symbolized in ASTM C 457 by T). It is the length of the
sum of the chords of the air voids divided by the number of voids encountered in
the traverse.

4. Specific surface (symbolized in ASTM C 457 by a). It is the surface area of the
average void divided by the volume of the average void. It is calculated from
the average chord. The unit involved can be expressed as squared units divided
by cubed units or as units to the minus 1 power. The higher values (higher void
surface area per void internal volume) indicate smaller voids. Small voids (with
shorter average chord) are desired because they disperse throughout the concrete
with small unprotected volumes of paste in between. If the same air-void
content was present in larger voids, the unprotected volumes of paste would be
much larger.

5. Spacing factor (symbolized in ASTM C 457 by L). It is calculated from the
specific surface, the percentage of air voids, and the percentage of paste (see 7.1)
that must be protected. It is expressed as a decimal value of the measurement
unit. The spacing factor is a theoretical measure of the average distance water,
ice, or expansive force must to travel in HCC before it contacts an air void, i.e.,
half the average distance between air voids. The smaller the spacing factor, the
more completely the air-void system can protect the concrete against deterioration
by freezing and thawing. Regardless of the ratio of air-void volume to paste
volume, the higher values for void frequency and the concomitant shorter average
chord length result in smaller spacing factors and a more desirable air-void
system.

6.3.2 Methods and Equipment

6.3.2.1 Overview

New methods and equipment are continually being devised to monitor and determine
the air-void parameters of hardened concrete. It is part of the job of the petrographer
to assess the value of new methods and equipment and decide which
method is of value in which situation and, therefore, which equipment is worthy of
a place in the budget of the organization. If the results of an air-void determination
are to be presented in court and the testimony of opposing expert witnesses will be
heard, any deviation from the principles of ASTM C 457 that has not been agreed
on by the client may invalidate the results of the analysis. Within an organization,
certain deviations from the strict interpretation of ASTM C 457 may be acceptable.

According to ASTM C 457, air-void system analyses can be efficiently performed
with several methods and kinds of equipment. Suitable equipment for the determination
of air-void parameters in hardened concrete includes, but is not necessarily
limited to, (1) linear traverse, (2) modified point-count, and (3) image analysis
equipment. In common, the types of equipment to be used permit or facilitate the
movement of the specimen of HCC on the stage of a microscope so that data may be
collected over the specified area and from the specified length of traverse. In common,
the data collected are:

1. The total length of traverse over which the determination is made. In the
modified point-count method, the total number of points examined and the distance
the equipment moves between the points are required for the calculations.

2. The portion of the traverse that is across air voids. In the linear traverse
method, this portion is the sum of the chord lengths across air voids; in the modified
point-count method, this portion is the number of points that occur in air voids
multiplied by the distance the equipment moves between points.

3. The number of voids occurring in the traverse examined. The accuracy of the
determination of the specific surface and spacing factor is completely dependent
on the accuracy of the count of the number of voids on the line traversed. In the
linear traverse method, the number of voids in the traverse is the number of
chords collected; in the modified point-count method, it is the number of voids
counted along the traverse line.

The procedures detailed in ASTM C 457 are those to be used with nonelectronic
types of equipment (see ASTM C 457, Figs. 2 and 5). When equipment is used that
includes automatic devices for moving the specimen, electric or electronic counters
or totalizers and calculators, or measuring devices, the equipment must allow adherence to the principles of ASTM C 457 and permit or perform the calculation of
the same parameters of the air-void system from the same data. The exact procedures
followed for the operation of the equipment must be those described and specified
by the fabricator of the equipment.

It is not known which type of equipment produces the most accurate results or how
accurate the results need to be. Point count is favored by those who need speed.
Linear traverse is favored by those who wish a record of the chord length distribution
for research purposes. Image analysis is favored by those who desire speed and
the ability to collect a lot of data and manipulate it on a computer in many different
ways. Image analysis can strain the equipment budget but requires less operator
time since the specimen is not examined by the human eye. Image analysis equipment
is not available at VTRC. Research laboratories will usually require either
point-count or image analysis equipment for speed in making routine determinations
and linear traverse equipment for its ability to determine chord length distribution
on a surface unaltered by the fillers and dyes required by image analysis.

>NOTE: The air-paste ratio method of calculation detailed in ASTM C 457 is to
be used ONLY (1) when proportions of the ingredients in the mixture are known
with some certainty, (2) it can be assumed that no change in mixture proportions
has occurred (e.g., retempering has not occurred; i.e., the amount of paste can be
closely calculated), AND (3) because of the lack of exposure of a generalized
specimen of the HCC or because of the extremely large size of the aggregate it is
impossible to obtain a specimen of the HCC for microscopical analysis with an
aggregate distribution that is representative of the placement. The air-paste ratio
calculations use the aggregate-paste ratio of the design of the mixture to
transform mathematically the air-paste ratio determined microscopically to
percentage air voids, specific surface, and spacing factor. In these situations, it is
convenient to select a specimen of HCC that is low in aggregate so that the
microscopist will not have to spend excess time moving over aggregate.

6.3.2.2 Linear Traverse

Using the linear traverse equipment (see Fig. 6-5), the operator tabulates the chord
lengths across all phases of interest and records them for later analysis (Walker,
1988). This sort of data permits the straightforward calculation of the void parameters
by the summing of the lengths of the chords and counting of each occurrence of

a phase. Because the calculations are extremely sensitive to errors made in the
determination of the number of voids traversed, the method of deciding whether a
void is or is not touched or transected by the line of traverse must be carefully
employed in any case of doubt. If the individual lengths of the chords of the air
voids are recorded and certain shape assumptions are made, a graphical representation
of the chord lengths will indicate the size distribution of the air voids. The
collection of the air-void data requires one pass of the microscope along the traverse
line. The data necessary for calculation of the paste content may be collected at the
same time or a separate determination can be made for the paste content. This procedure
is further discussed in 7.1.2. With some types of linear traverse equipment,
all the air-void parameters are automatically calculated; with others, the calculations
must be performed on a calculator or computer.

6.3.2.3 Point Count

With the point-count equipment (see Fig. 6-6), the operator records the type of substance
(air void, paste, or aggregate) appearing at the index point of the reticle at a
large number of points as provided by a click stop on the stage. The points may be
randomly distributed or regularly distributed on a randomly placed grid or a traverse
line. Data concerning the relative amounts of all the phases can be collected
from one pass along the traverse line. Calculation of voids per unit length, average
chord length, specific surface, and spacing factor usually requires that a second
pass along the traverse be used to count the number of voids occurring on the traverse
line. Although the majority of users of the point-count apparatus collect the
information concerning the abundance of paste during the same pass on which that
concerning the abundance of air is collected, they may sometimes find difficulty in

Figure 6-6 FULLY AUTOMATED EQUIPMENT FOR DETERMINING AIR-VOID PARAMETERS.
Either linear traverse or point-count software can be used to control the computer and the
motion of the stage.

distinguishing the exact paste-aggregate boundary and might wish to consider a
separate pass over a lightly etched surface to collect these data (see 7.1.2). The
air-void parameters may be calculated by the analysis equipment or separately calculated
using a calculator or computer.

6.3.2.4 Image Analysis

Image analysis equipment (see Fig. 6-7) requires that the specimen be specially prepared
so that each of the three major phases of interest (voids, paste, and aggregate)
is a distinct tone (e.g., white, black, and medium gray). The presence and
shape of areas of the three selected tones are determined by electronic eye, and the
data are automatically recorded, sorted, and calculated. The specimen preparation
methods for image analysis can be exacting and make the surface used useless for
ordinary stereomicroscopic examination (as described in Chapter 8). If it is desirable
to examine the distribution of phases with the human eye later, when the specimen is
sawed, the surface facing the surface that is to be colored and filled should
be finely lapped for microscopical evaluation. Both of these surfaces should be kept
intact and safe until all controversy regarding the concrete is over. Automatic systems
that require filling the voids (thus hiding their interior surface) cannot be used
to make certain distinctions possible by a human operator. The human operator
can often mentally reconstruct what the surface examined would have been if this
or that flaw had not occurred. The human operator can judge if a void observed is
an air void, a fly-ash cenosphere, or the hole left where a small round grain of sand
has fallen out. These distinctions are generally possible by study of the reaction
products and the luster of the interior of the void.

Figure 6-7 IMAGE ANALYSIS EQUIPMENT. The instrument is shown in the process of analyzing
the air-void system of a slice of concrete. The screen in the background shows the progress of the
analysis. (Photograph by R. H. Howe, courtesy of PennDot.)

6.3.2.5 Other Considerations

At VTRC, it has long been recognized that the accuracy of a linear traverse determination
of the air-void parameters is as dependent or more dependent on the number
of voids encountered and measured along the traverse as it is on the length of the
traverse. Once 1,000 voids have been measured and counted, the results from the
data obtained subsequently change very little. Snyder, Hover, and Natesaiyer
(1991) made an analytical investigation of the effect of the number of voids and the
length of the traverse on the minimum expected error that can be encountered in a
linear traverse determination of the void parameters in hardened HCC. Their work
supports our belief that little additional accuracy is achieved if the determination
includes more than 1,000 voids and that almost no additional accuracy is achieved
with more than 2,000 voids.

For intradepartmental purposes, for ordinary determination of the air-void parameters,
it has been our practice to estimate visually the number of voids per unit
length of traverse, consider how long a traverse is required in order to obtain data
on 1,000 voids, and then plan how to spread the traverse length evenly over the
specimen surface available. Under circumstances when ASTM C 457 requires 100
in. of traverse, we estimate that only 50 to 70 in. is required for the collection of
data from 1,000 voids in ordinary within-specification concrete. Should some legal
controversy arise concerning the subject concrete, any traverse length deficiency
can be made up by collection of data from the lacking inches of traverse (also evenly
distributed over the surface). If the traverse direction and starting point are randomly
chosen, in both cases, the randomness of the data collection will be maintained. It is
our view that spreading the data collection area over as large a surface
area as possible so that any irregularities of void distribution (any clumping or
areas devoid of voids) become part of the data recorded and examined is more important
than the length of the traverse line.

The corollary is that if the number of voids is very small due to low air content or
large voids, the length of traverse recommended in ASTM C 457 is probably not sufficient
to obtain accurate air-void parameter data (Snyder, Hover, & Natesaiyer,
1991). Under such circumstances and with borderline compliance with specifications,
it may be wise to use an additional length of traverse to ensure accuracy In
most cases, the small specific surface and large spacing factors caused by the lack of
sufficient small voids will decisively indicate lack of compliance with the specifications.

The method of deciding whether a void is or is not touched or transected by the line
of traverse must be a simple rule that is firmly adhered to throughout any particular
analysis. Pleau, Plante, Gagne, and Pigeon (1990), using the point-count method,
recommended: "A simple way to guarantee a random choice is to systematically
choose the constituent located in a given quadrant (of the field viewed), say the upper
left corner of the cross-hairs." A similar method can be devised for whatever
type reference point, reticle, and counting method are in use. Other researchers
(Mather for one [1989]) have suggested that points in dispute be collected in a separate
register and later distributed to the totals of the constituents in the same proportion
as are the data concerning which there is no dispute.

6.3.3 Preparation of Specimens

The importance of the proper preparation of the surface of the slice of concrete cannot
be overemphasized (see Figs. 5-2 and 5-3). In most laboratories, specimens are
prepared by skilled, highly trained technicians. A poorly prepared specimen can
cause a determination of the percentage of air present in a specimen to deviate from
the true value by as much as 2 percentage points (20% to 50% of the true value). A
rough surface makes it impossible to detect small voids. This will have the effect of
lowering the detected percentage of air, decidedly lowering the specific surface, and.
thus raising the spacing factor. Quantitative determination on a surface that is
undercut and wherein the edges of the voids have been chipped or worn away can
provide data that indicate the presence of more air than really exists.

The preparation methods used by Pleau et al. (1990) can be questioned. They advocate
soaking the slices (slabs) in water for a few days, presumably to complete the
hydration and produce a more stable material for lapping. (Chapter 5 has several
suggestions for the treatment of weak or immature concrete before lapping.) However,
water can wash away reaction products, liquefy expansive alkali-silica gels,
dissolve calcium hydroxide crystals, loosen aggregates in their sockets, change the
appearance of the inner void surfaces, and weaken thin paste walls between voids.
The void walls and remnants of void walls serve to define the void boundaries and
facilitate the recognition of the individual void structures. If the inner void surfaces
are in their original condition, the luster, surface texture, and asperities on these
surfaces can help distinguish the differences among entrained voids, entrapped
voids, water channelways, and aggregate sockets. Thus, water should not be used
in sample preparation. It is common practice in concrete laboratories to use a saturated
solution of calcium hydroxide as the water bath whenever specimens of concrete
are soaked in water (for testing absorption etc.) to prevent weakening the concrete
by dissolving of the contained calcium hydroxide that is an important part of
the structure of most concretes. The calcium hydroxide solution may have undesirable
effects on specimens prepared for microscopical analyses and is not recommended for
this purpose.

Each method of producing a finely lapped specimen surface for microscopical study
will probably produce different effects on different types of concrete (different
water-cement ratios, different kinds of aggregates, different degrees of maturity and
deterioration).

In certain concretes in which the shape of the air voids has become distorted (see
Appendix D), all manner of overlaps and crushing of voids may occur; the operator
should be alert and ready to record the data for each void in a logical and consistent
manner.

6.3.4 Technician Considerations

The linear traverse and modified point-count methods are tedious and hard on the
eyes. A single determination of the air-void parameters of a concrete by means of
the linear traverse method can take up to 7 hr, depending on the size and quantity
of the voids. A technician cannot spend more than 4 hr a day doing this sort of
work on a day-to-day basis. Everyone who has tried has found that the ability of
their eyes to focus has deteriorated on the following day. The training and keeping
of good microscopical technicians can be a major undertaking requiring tact, skill,
understanding, and a flexible schedule of rest periods.

Image analysis systems do not require that the operator be with the equipment
after the initial adjustment; thus, eye fatigue and the need to train technicians to perform
microscopical analyses are eliminated.

The following points are important considerations in the hiring and training of
technicians for the microscopical analysis of air-void systems:

Try to avoid hiring operators for linear traverse and point-count determinations
of air-void parameters who do not have good binocular vision.

Keep available standard specimens of concrete with a range of different types of
air-void systems. Air-void contents of 2% to 14% are recommended. These
should be specimens that have been analyzed by a number of different operators.
The results previously obtained should be kept in a secure place by the supervisor.
Each new operator who is trained for this work should be tested on the
standard specimens, and training should continue until the results of the new
operator are comparable to the range of results recorded in the past.

Make sure that each operator knows how to adjust the positioning of the specimen
so that it is flat and so that the specimen can move under the microscope
and remain in the same focal plane. This procedure can be a tedious nuisance
and may be neglected if its importance is not sufficiently emphasized during the
training of the operators.

Make sure that each operator knows how to adjust the binocular vision spacing;
the height of his or her chair; and any other items available for greatest visual
acuity, comfort, and convenience. The operator must understand that these adjustments
are not emphasized for his or her personal comfort but rather because
proper adjustment adds to the accuracy of the determination. An operator suffering
from a headache or backache is not as able to produce good data as a comfortable,
healthy operator.

Make sure that the operators understand the need for good focus and how to
achieve good focus on the reticle for their main eye and simultaneous good focus
on the specimen for both eyes. Each person has one eye that looks straight
ahead (the main eye). The other eye observes things at an angle. Whenever an
optical technique requires a reticle in one eyepiece of a binocular system, the retide
should be placed in the lens system used by the main eye.

If the microscope is used by more than one person, make it a routine practice for
each operator when beginning work to check the focus of the reticle and the focus
of the surface being examined. Emphasize that the surface should be in focus
throughout an entire traverse across the specimen. If focus is lost, errors
will be great and the ability to judge the type and origin of a flaw in the finish of
the surface being examined will be seriously diminished.

Observe the actions of the operators and determine if they are following instructions.
From time to time, have the work of the operators checked by having
another operator redo a specimen, an operator redo a specimen done some
months ago, or an operator redo one or more of the standard training specimens.

Teach operators that great care must be taken to include in the count every void
crossed by the traverse. The air-void count should be performed slowly and accurately.
Very small voids and voids that are just barely touched by the traverse
line must be counted. When the linear traverse procedure is used, it may be necessary
to slow the motion along the traverse almost to a stop (if not completely)
to register very small voids in the count. If it is realized that a void with an essentially
zero chord length (because the traverse line is tangent to the void or
because the void is very tiny) has not been counted, it is possible with some
equipment to bring the motion along the traverse to a halt (so that zero chord
length is recorded) and press momentarily the button that registers the presence
of the appropriate void type. The location of a void along the traverse line is not
a matter of concern, and the operator can record it anywhere. In the modified
point-count method, no automatic motion is usually used while air voids existing
along the traverse line are counted; therefore, this error will not be made in the
same way.

6.4 CLASSIFICATION OF VOIDS

6.4.1 Overview

Determination of the abundance of the various types of voids is very useful in concrete
research. It can make data available that can change various practices in the
mixing and placing of HCC. As an example, it was once thought that the speed of
the screed pulling the vibrators through freshly placed concrete did not affect the
degree of consolidation. This did not seem logical to some. The Ballenger Construction
Co. of North Carolina set up a series of test sections of pavement in which the
speed of the screed was carefully controlled. A detailed petrographic laboratory
analysis of the abundance of the various sizes of voids in the air-void system of 24
cores that had been removed from these test sections showed that there is a good
inverse relationship between the speed of the screed and the degree of consolidation
(Walker, 1972a). As a result, the maximum speed of the screed is now limited in
many specifications.

The quantitative determination of the abundance of various types of voids can be an
important part of the petrographic analysis of a specimen. In the normal usage of
linear traverse equipment in the VTRC petrographic laboratory, the abundance of
each of the three types of air voids (entrained, entrapped, and water formed) is routinely
determined (see Table 6-1). With equipment designed for this purpose, this
determination may be performed concurrently with the determination of the other
parameters of the air-void system.

In a finely lapped slab or a thin section, the size and shape of voids can be used as
indicators of void origin and type. The luster and texture of the interior of the voids
may sometimes be used in the recognition of voids caused by accumulations of water
and passageways for water. The properties on which distinctions may be made
between the various types of voids are arbitrary and may vary from one laboratory
to the next. Because these distinctions are made on the appearance of a void on the
surface of a slice (where the third dimension of the void cannot be seen), many large
voids will be classified as entrained voids when they are really entrapped voids. As
indicated in Figure 6-8, a small section through a large void can, in two dimensions,
be indistinguishable from a large section through a small void. A cross section that
is larger than the defined maximum for entrained voids must be a section of an entrapped
air void or a water-formed void. A large number of large cross sections indicates a
large number of large voids.

Figure 6-8 ILLUSTRATION OF VARIOUS SIZES OF SECTIONS THAT MAY BE
EXPRESSED ON RANDOMLY PLACED PLANE

6.4.2 Distinguishing Between Entrapped Voids Caused by Air
and Those Caused by Water

The petrographer can often distinguish between entrapped voids caused by water
pockets and entrapped voids caused by air pockets. The estimate will necessitate
careful observation and some extrapolation. In general, the interior surface of an
air void will appear smoother, sometimes even shiny. A water void will usually
have a dull interior that appears to have had small particles and precipitates
deposited on it. In the case of water-formed voids, the shapes of the bounding aggregate
particles are often visible in the interior of the void. Water voids may have an interior
showing water movement patterns, may be interconnected bleed water voids, or
may show by nature of the internal deposits and asperities and by position that
they are water pockets trapped by aggregate particles.

6.4.3 Determination of Size Break Point Between Entrained
and Entrapped Voids

The determination of the size break point between entrained and entrapped voids
varies from laboratory to laboratory and must be interpreted in light of the method
of measurement. For example, if the voids whose maximum chord on the surface
examined is less than 1 mm are defined as entrained voids, then some voids whose
true diameter is larger but not observable because the diameter is not in the plane
of observation will be classified as entrained voids. The petrographer should maintain
a clear idea of the meaning of the methods of determination in the size sorting
of the voids.

A random line of traverse through HCC has a greater chance of traversing a large
void than a small one. The probability ratio is as the ratio of their volumes (see
Fig. 6-9). The calculations detailed in ASTM C 457 are designed to be used on the
sums of the chord lengths and on the count of the voids regardless of the desirability
or relative amounts of the various sizes. If the large voids are not counted and
measured as part of the overall determination of the void system (suggested by
Sommer [1979]), the control against large voids provided by the determination of
the specific surface and spacing factor will have been blocked and the apparent precision
of the method spuriously improved (Walker, 1980).

Unless the method and the criteria used to obtain data concerning void size are
rooted in statistics, the data are only rules of thumb and valid only when compared
with data obtained by the same methods. Calculations can be made on the distribution
of the void sizes from chord data if certain assumptions are made concerning
shape, heterogeneity, and isometric distribution of air voids.

At VTRC, the diameter of the section of the void as seen on the finely lapped surface
examined must be equal to or less than 1 mm for the void to be considered an entrained
void. In other laboratories, the length of the chord on the traverse line
across the void is the parameter measured. The latter method makes it possible to
set up an automatic electronic classification and counting system for entrained versus
entrapped voids. In some European laboratories, the chord must be 0.3 mm or
less for the void to be considered an entrained void (Wilk, Dobrolubov, Romer,
1974). A void viewed in a lapped surface may be transected by the surface either
above or below its true diameter, and there is no known way to measure an actual
internal diameter. Efforts have been made to peer into a void to try to get an estimate
of the true diameter, but in my view these efforts serve only to confuse the issue.

Figure 6-9 TWO EQUALLY SPACED ARRAYS OF VOIDS. Each is crossed by a randomly
oriented plane. There is the same number of voids in a unit area in each array. Note that the plane
touches more voids (see arrows) when the voids are large than when the voids are small.

In many laboratories, decisions on individual void size are made on the lapped
surface as viewed. A large void, more than 1 mm in diameter, may be so oriented
that the surface examined truncates only a small portion of the void, the extreme
top or bottom, when considered from the finely lapped surface. Thus, there will always
exist a larger proportion of large voids than can be recognized on the surface
examined (see Fig. 6-8).

6.4.4 Procedures

The procedures given here are for the linear traverse method when the chord
lengths are collected by an operator depressing an electric recording device and
either a paper printout is produced or three collecting devices are available (Walker,
1988). The point-count method does not survey every void on the traverse during
the percentage portion of the examination and, therefore, does not allow a classification
of every void. Image analysis procedures are not discussed in detail because
such equipment is not available at VTRC.

1. Examine each void when the void's leading edge comes to the index
point (usually the center of the cross hairs), and determine which type of
void is present. With the wide-angle lenses and a magnification of 100X or less,
voids of less than 1 mm in diameter will be completely visible in the field of view.
Most voids can be classified at a glance as either entrained, entrapped, or water
formed. When borderline cases occur, use a finely marked metric ruler on the slice,
in the field of the microscope, to determine void size (at low magnification, an eye-
piece micrometer may be used).

2. Record the presence of the void and the length of the chord across it in
the usual manner for linear traverse (by pushing down a button and holding it
down until the trailing edge of the void is at the index point). If three buttons are
available for the three types of voids, each with their own totalizing devices that
separately measure, total, and count the voids, depress the button appropriate for
the type of void determined in step 1. If the chord lengths are recorded by using
only one button and are individually printed on paper and the void encountered is
not an entrained void, stop the motion of the traverse stage and mark the paper
tape at the chord measurement with a symbol to indicate the classification of the
void measured. Continue with the analysis; repeat from step 1 for each void.

3. When the analysis is complete, add the lengths of the chords for each
type of void (if not added by the linear traverse device employed) and report the
percentage by volume and the count (individual voids per specified inches of traverse)
of each in the total concrete.

6.5 MEANING OF AIR-VOID PARAMETERS

The major parameters of the air-void system are interdependent. Some specifiers of
concrete will require only that the air-void content is within certain limits; others
will require that the spacing factor be below a certain limit or the specific surface be
above a certain limit. Because one-sided limits on spacing factor and specific surface
do not indicate the presence of a too high air-void content, the air-void content
should be required to be within an upper as well as a lower limit.

The following are some of the issues the air-void parameters will determine:

Resistance to deterioration caused by freezing and thawing. When ASTM C 457
is followed with care and the report is as instructed therein, the numerical data
obtained will clearly indicate the ability of the cement paste in the concrete
represented by the specimen to resist the destructive forces of freezing and thawing.
A specific surface of more than 600 in.-1 (in.2/in.3) and a spacing factor of less
than 0.008 in. indicate an HCC with a paste having an air-void system of the
type that will resist severe winter weather conditions in a mature HCC containing
few microcracks. Certain concretes with a very low permeability (and usually a very high
strength) may resist the forces of freezing and thawing without
meeting these requirements either because they never get critically saturated or
(less likely) they lack sufficient freezable water when saturated.

Use of specific admixtures. The petrographer is often asked if certain admixures
or an excess amount of a certain admixture has been used in a specific concrete.
Extraordinarily low spacing factors accompanied by a high specific surface can
indicate either excessive air-entraining agent or (if the total air content
is within specifications) the use of a highly specialized admixture. During the
first trials of some of the high-range water reducers, the paste was exceedingly
compact but many of the voids were large. This created a specific texture, as
illustrated in Figure 6-10. The high-range water reducers used in present-day
mixtures do not create such a high content of large voids, but some concrete with
this texture is still in service and requests to examine these concretes can still
come in.

Flaws. An unusually large number of voids that appear to have held water
when the concrete was fresh indicate flaws in either the proportioning or the
workmanship. An abundance of entrapped voids indicates either poor consolidation
or early loss of slump.

Figure 6-10 TYPE OF VOIDS AND PASTE TEXTURE PRODUCED BY EARLY TYPES OF
HIGH-RANGE WATER REDUCERS. Such large voids do not add to the resistance of the concrete
to freezing and thawing but do lower the compressive strength. The scale is in millimeters.

The data concerning the distribution of the types of voids as detailed in 6.4 can be
used to discover certain placement conditions, such as the efficiency of the consolidation
and reasons for various nonstandard conditions (such as low compressive
strength or high permeability). Sometimes, when hand-held vibrators are used,
there is excess entrapped air because of the persistent but erroneous belief that the
specified amount of vibration will cause a loss of a portion of the required entrained
air. It has been demonstrated that vibration even 2 or 3 times as long as the required
amount does not reduce the entrained air in properly proportioned mixtures
(R. H. Howe, personal communication, October 24, 1991). Although the maximum
allowable quantity of large voids and the mathematical expression of a large quantity
of large voids in a high spacing factor and low specific surface are not parameters
required by the specifications of VDOT (thus difficult to argue in a court of
law), it is important to consider these parameters and be able to explain their
meaning. In general, it is much easier to talk about a large quantity of large voids
than to explain the mathematical derivation of the specific surface and the spacing
factor. If the percentage of the large voids exceeds 1.5%, it is considered high. More
than 2% is considered excessive, and explanations for the prevalence of the large
voids is sought. Does the concrete appear to have been retempered? Improperly
mixed or consolidated? What is the reason?